linux: fix remount readonly in a user namespace
linux: fix remount readonly in a user namespace

if we are remounting root read only when in a user namespace, make
sure the existing flags (e.g. MS_NOEXEC, MS_NODEV) are maintained
otherwise the mount fails with EPERM.

Signed-off-by: Giuseppe Scrivano <20002b7b24c75512093a71a58285826d5c504fca@redhat.com>

rhatdan/runc,kolyshkin/runc,rhatdan/runc,estesp/runc,estesp/runc,opencontainers/runc,opencontainers/runc,dqminh/runc,dqminh/runc,opencontainers/runc,kolyshkin/runc,estesp/runc,rhatdan/runc,kolyshkin/runc,xiaochenshen/runc,dqminh/runc,xiaochenshen/runc,opencontainers/runc,xiaochenshen/runc,kolyshkin/runc,rhatdan/runc,dqminh/runc